A conventional automatic transmission for a vehicle is known that induces a gear change rotational speed from a predetermined map based on a vehicle speed and a throttle opening and automatically performs a gear change operation based on the gear change rotational speed. There has also been known a transmission control device which, when it is determined that the deceleration of a vehicle body exceeds a predetermined value using a deceleration brake which is mounted on a wheel or the like, performs the shift-down to a gear change step which can obtain larger engine brake irrespective of the above-mentioned predetermined map.
For example, JP-A-9-42433 1 discloses such a transmission control device which changes a predetermined value of the deceleration which becomes a criterion for determining whether the shift-down is performed or not based on a vehicle speed. According to this transmission control device, for example, by setting the predetermined value of the deceleration, which is applied to the traveling at 80 km/h, larger than the predetermined value which is applied to the traveling at 50 km/h, the shift-down is not performed unless the larger deceleration is generated when the vehicle body is decelerated from the traveling at 80 km/h, thus preventing the generation of a large transmission shock attributed to the shift-down from the high-speed traveling.